1. Field of the Invention
The present invention relates to a subjective ophthalmic instrument for examining eyes. More specifically, the present invention pertains to a subjective ophthalmic instrument by which the operator can observe an image of optotypes formed on the patient's fundus in order to examine the eye refractive power, the eye optical system and the optic nerve of the patient.
2. Description of the Prior Art
Conventionally, ophthalmic instruments are divided into two types, namely, objective ophthalmic instruments and subjective ophthalmic instruments. In an objective ophthalmic instrument, an image of a split mark such as a split slit is projected on the patient's fundus so that the refractive power of the patient's eye is measured in accordance with amount of image splitting. In a subjective ophthalmic instrument, the patient watches optotypes such as a Landholt ring through a correcting lens system, which is adjusted in accordance with the patient's response so that the patient can see the optotypes clearly. The patient's refractive power is measured in terms of the amount of adjustment.
The objective ophthalmic instrument has an advantage in that the patient's response is not necessary, while the subjective ophthalmic instrument has an advantage in being able to directly measure patient's precise refractive power. With the subjective ophthalmic instrument, however, the operator cannot examine how the image of the optotypes are produced on the patient's fundus so that the measurement is performed through the response of the patient. Since the patient's response sometimes includes errors, quick and precise examination of the eye cannot be expected with the subjective ophthalmic instrument.
Furthermore, in the subjective ophthalmic instrument, when the image of the optotypes formed at the fundus is examined while the patient watches the image of optotypes the operator cannot watch the image of the optotypes precisely, because the image is blurred because of the low reflection efficiency of the fundus and low conversion efficiency of the photo-electric element used.
In view of the frequent incorrectness of the patient's response, it has become common to carry out refractive power measurement by a rough measurement using an objective ophthalmic instrument and subsequently referring to the rough refractive power data in carrying out a precise measurement using a subjective ophthalmic instrument provided in the same objective ophthalmic instrument or independently of it.
In the conventional subjective ophthalmic instrument, when the patient cannot see the optotypes clearly at any adjustment of the correcting lens system, the operator cannot judge whether the patient's eye suffers from a disease affecting his eye optical system between the cornea and the retina or a disease affecting his optic nerve extending to the retina.